Re: [PATCH v2 2/3] KVM: x86: introduce KVM_MEM_PCI_HOLE memory

[Michael S. Tsirkin]

On Thu, Aug 13, 2020 at 07:31:39PM -0700, Sean Christopherson wrote:
> On Fri, Aug 07, 2020 at 04:12:31PM +0200, Vitaly Kuznetsov wrote:
> > PCIe config space can (depending on the configuration) be quite big but
> > usually is sparsely populated. Guest may scan it by accessing individual
> > device's page which, when device is missing, is supposed to have 'pci
> > hole' semantics: reads return '0xff' and writes get discarded. Compared
> > to the already existing KVM_MEM_READONLY, VMM doesn't need to allocate
> > real memory and stuff it with '0xff'.
> > 
> > Suggested-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> > ---
> >  Documentation/virt/kvm/api.rst  | 18 ++++++++++-----
> >  arch/x86/include/uapi/asm/kvm.h |  1 +
> >  arch/x86/kvm/mmu/mmu.c          |  5 ++++-
> >  arch/x86/kvm/mmu/paging_tmpl.h  |  3 +++
> >  arch/x86/kvm/x86.c              | 10 ++++++---
> >  include/linux/kvm_host.h        |  3 +++
> >  include/uapi/linux/kvm.h        |  2 ++
> >  virt/kvm/kvm_main.c             | 39 +++++++++++++++++++++++++++------
> >  8 files changed, 64 insertions(+), 17 deletions(-)
> > 
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index 644e5326aa50..dc4172352635 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -1241,6 +1241,7 @@ yet and must be cleared on entry.
> >    /* for kvm_memory_region::flags */
> >    #define KVM_MEM_LOG_DIRTY_PAGES	(1UL << 0)
> >    #define KVM_MEM_READONLY	(1UL << 1)
> > +  #define KVM_MEM_PCI_HOLE		(1UL << 2)
> >  
> >  This ioctl allows the user to create, modify or delete a guest physical
> >  memory slot.  Bits 0-15 of "slot" specify the slot id and this value
> > @@ -1268,12 +1269,17 @@ It is recommended that the lower 21 bits of guest_phys_addr and userspace_addr
> >  be identical.  This allows large pages in the guest to be backed by large
> >  pages in the host.
> >  
> > -The flags field supports two flags: KVM_MEM_LOG_DIRTY_PAGES and
> > -KVM_MEM_READONLY.  The former can be set to instruct KVM to keep track of
> > -writes to memory within the slot.  See KVM_GET_DIRTY_LOG ioctl to know how to
> > -use it.  The latter can be set, if KVM_CAP_READONLY_MEM capability allows it,
> > -to make a new slot read-only.  In this case, writes to this memory will be
> > -posted to userspace as KVM_EXIT_MMIO exits.
> > +The flags field supports the following flags: KVM_MEM_LOG_DIRTY_PAGES,
> > +KVM_MEM_READONLY, KVM_MEM_PCI_HOLE:
> > +- KVM_MEM_LOG_DIRTY_PAGES: log writes.  Use KVM_GET_DIRTY_LOG to retreive
> > +  the log.
> > +- KVM_MEM_READONLY: exit to userspace with KVM_EXIT_MMIO on writes.  Only
> > +  available when KVM_CAP_READONLY_MEM is present.
> > +- KVM_MEM_PCI_HOLE: always return 0xff on reads, exit to userspace with
> > +  KVM_EXIT_MMIO on writes.  Only available when KVM_CAP_PCI_HOLE_MEM is
> > +  present.  When setting the memory region 'userspace_addr' must be NULL.
> > +  This flag is mutually exclusive with KVM_MEM_LOG_DIRTY_PAGES and with
> > +  KVM_MEM_READONLY.
> >  
> >  When the KVM_CAP_SYNC_MMU capability is available, changes in the backing of
> >  the memory region are automatically reflected into the guest.  For example, an
> > diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
> > index 17c5a038f42d..cf80a26d74f5 100644
> > --- a/arch/x86/include/uapi/asm/kvm.h
> > +++ b/arch/x86/include/uapi/asm/kvm.h
> > @@ -48,6 +48,7 @@
> >  #define __KVM_HAVE_XSAVE
> >  #define __KVM_HAVE_XCRS
> >  #define __KVM_HAVE_READONLY_MEM
> > +#define __KVM_HAVE_PCI_HOLE_MEM
> >  
> >  /* Architectural interrupt line count. */
> >  #define KVM_NR_INTERRUPTS 256
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index fef6956393f7..4a2a7fface1e 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3254,7 +3254,7 @@ static int kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, gfn_t gfn,
> >  		return PG_LEVEL_4K;
> >  
> >  	slot = gfn_to_memslot_dirty_bitmap(vcpu, gfn, true);
> > -	if (!slot)
> > +	if (!slot || (slot->flags & KVM_MEM_PCI_HOLE))
> 
> This is unnecessary since you're setting disallow_lpage in
> kvm_alloc_memslot_metadata().
> 
> >  		return PG_LEVEL_4K;
> >  
> >  	max_level = min(max_level, max_huge_page_level);
> > @@ -4105,6 +4105,9 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
> >  
> >  	slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
> >  
> > +	if (!write && slot && (slot->flags & KVM_MEM_PCI_HOLE))
> 
> I'm confused.  Why does this short circuit reads but not writes?
> 
> > +		return RET_PF_EMULATE;
> > +
> >  	if (try_async_pf(vcpu, slot, prefault, gfn, gpa, &pfn, write,
> >  			 &map_writable))
> >  		return RET_PF_RETRY;
> > diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> > index 5c6a895f67c3..27abd69e69f6 100644
> > --- a/arch/x86/kvm/mmu/paging_tmpl.h
> > +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> > @@ -836,6 +836,9 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gpa_t addr, u32 error_code,
> >  
> >  	slot = kvm_vcpu_gfn_to_memslot(vcpu, walker.gfn);
> >  
> > +	if (!write_fault && slot && (slot->flags & KVM_MEM_PCI_HOLE))
> > +		return RET_PF_EMULATE;
> > +
> >  	if (try_async_pf(vcpu, slot, prefault, walker.gfn, addr, &pfn,
> >  			 write_fault, &map_writable))
> >  		return RET_PF_RETRY;
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index dc4370394ab8..538bc58a22db 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -3515,6 +3515,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> >  	case KVM_CAP_EXCEPTION_PAYLOAD:
> >  	case KVM_CAP_SET_GUEST_DEBUG:
> >  	case KVM_CAP_LAST_CPU:
> > +	case KVM_CAP_PCI_HOLE_MEM:
> >  		r = 1;
> >  		break;
> >  	case KVM_CAP_SYNC_REGS:
> > @@ -10114,9 +10115,11 @@ static int kvm_alloc_memslot_metadata(struct kvm_memory_slot *slot,
> >  		ugfn = slot->userspace_addr >> PAGE_SHIFT;
> >  		/*
> >  		 * If the gfn and userspace address are not aligned wrt each
> > -		 * other, disable large page support for this slot.
> > +		 * other, disable large page support for this slot. Also,
> > +		 * disable large page support for KVM_MEM_PCI_HOLE slots.
> >  		 */
> > -		if ((slot->base_gfn ^ ugfn) & (KVM_PAGES_PER_HPAGE(level) - 1)) {
> > +		if ((slot->flags & KVM_MEM_PCI_HOLE) || ((slot->base_gfn ^ ugfn) &
> > +				      (KVM_PAGES_PER_HPAGE(level) - 1))) {
> >  			unsigned long j;
> >  
> >  			for (j = 0; j < lpages; ++j)
> > @@ -10178,7 +10181,8 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
> >  	 * Nothing to do for RO slots or CREATE/MOVE/DELETE of a slot.
> >  	 * See comments below.
> >  	 */
> > -	if ((change != KVM_MR_FLAGS_ONLY) || (new->flags & KVM_MEM_READONLY))
> > +	if ((change != KVM_MR_FLAGS_ONLY) ||
> > +	    (new->flags & (KVM_MEM_READONLY | KVM_MEM_PCI_HOLE)))
> >  		return;
> >  
> >  	/*
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 989afcbe642f..de1faa64a8ef 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -1081,6 +1081,9 @@ __gfn_to_memslot(struct kvm_memslots *slots, gfn_t gfn)
> >  static inline unsigned long
> >  __gfn_to_hva_memslot(struct kvm_memory_slot *slot, gfn_t gfn)
> >  {
> > +	/* Should never be called with a KVM_MEM_PCI_HOLE slot */
> > +	BUG_ON(!slot->userspace_addr);
> 
> So _technically_, userspace can hit this by allowing virtual address 0,
> which is very much non-standard, but theoretically legal.  It'd probably be
> better to use a value that can't possibly be a valid userspace_addr, e.g. a
> non-canonical value.
> 
> > +
> >  	return slot->userspace_addr + (gfn - slot->base_gfn) * PAGE_SIZE;
> >  }
> >  
> 
> ...
> 
> > @@ -2318,6 +2338,11 @@ static int __kvm_read_guest_page(struct kvm_memory_slot *slot, gfn_t gfn,
> >  	int r;
> >  	unsigned long addr;
> >  
> > +	if (unlikely(slot && (slot->flags & KVM_MEM_PCI_HOLE))) {
> > +		memset(data, 0xff, len);
> > +		return 0;
> > +	}
> 
> This feels wrong, shouldn't we be treating PCI_HOLE as MMIO?  Given that
> this is performance oriented, I would think we'd want to leverage the
> GPA from the VMCS instead of doing a full translation.
> 
> That brings up a potential alternative to adding a memslot flag.  What if
> we instead add a KVM_MMIO_BUS device similar to coalesced MMIO?  I think
> it'd be about the same amount of KVM code, and it would provide userspace
> with more flexibility, e.g. I assume it would allow handling even writes
> wholly within the kernel for certain ranges and/or use cases, and it'd
> allow stuffing a value other than 0xff (though I have no idea if there is
> a use case for this).

I still think down the road the way to go is to map
valid RO page full of 0xff to avoid exit on read.
I don't think a KVM_MMIO_BUS device will allow this, will it?


> Speaking of which, why do writes go to userspace in this series?
> 
> > +
> >  	addr = gfn_to_hva_memslot_prot(slot, gfn, NULL);
> >  	if (kvm_is_error_hva(addr))
> >  		return -EFAULT;
> > -- 
> > 2.25.4
> >